Methods and systems for processing a device, methods and systems for modeling same and the device

ABSTRACT

A method and system for locally processing a predetermined microstructure formed on a substrate without causing undesirable changes in electrical or physical characteristics of the substrate or other structures formed on the substrate are provided. The method includes providing information based on a model of laser pulse interactions with the predetermined microstructure, the substrate and the other structures. At least one characteristic of at least one pulse is determined based on the information. A pulsed laser beam is generated including the at least one pulse. The method further includes irradiating the at least one pulse having the at least one determined characteristic into a spot on the predetermined microstructure. The at least one determined characteristic and other characteristics of the at least one pulse are sufficient to locally process the predetermined microstructure without causing the undesirable changes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 60/279,644, filed Mar. 29, 2001, entitled “Method and System forSevering Highly Conductive Micro-Structures.” This application isrelated to U.S. patent application Ser. No. 10/107,028, filed on thesame day as this application, entitled “Method and System for ProcessingOne or More Microstructures of a Multi-Material Device.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of laser processing methodsand systems, and specifically, to laser processing methods and systemsfor processing microstructures formed on substrates. This invention isparticularly applicable, but not limited to, laser repair of redundantsemiconductor memory devices.

2. Background Art

In the repair of memory integrated circuits such as DRAMs and laserprogramming of high-density logic devices, the use of new materials,such as aluminum, gold, and copper, coupled with the small geometry ofthese devices, make the problem of link removal difficult. The newmaterials are typically metals or highly conductive composites havingreflectivity that is well over 90% in the visible and near infraredwavelength regions. Aluminum, for example, reflects greater than 90% ofthe laser energy over the range from the UV through to the nearinfrared. Gold and copper reflects even more strongly in the nearinfrared, the wavelengths of choice used by most of the lasers repairingmemories in production.

Further, economics and device performance have driven the size for theDRAMs and logic devices to very small physical dimensions. Not only arethe devices small, but the interconnects and links thickness have alsodecreased dramatically in recent years.

Thermal laser processing of links relies on the differential thermalexpansion between the oxide above the link and the link itself. Thisdifferential expansion results in a high pressure build-up of the moltenlink contained by the oxide. The oxide over the link is necessary tocontain the link in a molten state long enough to build-up sufficientpressure to crack the oxide and explosively expel the link material. Ifthe pressure is too low, the link will not be removed cleanly.Alternative laser wavelengths and laser control strive to increase thelaser “energy window” without damaging the substrate and materialcontiguous to the link.

Descriptions of an all-copper, dual-Damascene process technology can befound in “Benefits of Copper—Copper Technology is Here Today in WorkingDevices,” NOVELLUS DAMASEUS, Dec. 20, 2001; and “PreventingCross-Contamination Caused By Copper Diffusion and Other Sources,” P.Cacouvis, MICRO, July 1999.

FIGS. 2 a and 2 b illustrate prior art laser processing of multi-layerstructure wherein a target structure is located in proximity to asubstrate, with a q-switched pulse 20 from a conventional solid statelaser 21 irradiating and overfilling a target structure 23. A laser spotsize is typically significantly larger than the (target) link size whichrelaxes precision positioning requirements. A laser wavelength istypically selected based on substrate 27 (commonly Silicon) transmissionso as to allow for higher peak laser power or other system and processvariations. In certain cases, a layer 28,25 absorption coefficient iscontrolled (e.g., as a transition or protective layer) and/or awavelength selected wherein substrate damage is avoided.

Further information is available regarding link blowing methods andsystems, including material processing, system design, and device designconsiderations, in the following representative U.S. patents andpublished U.S. applications: U.S. Pat. Nos. 4,399,345; 4,532,402;4,826,785; 4,935,801; 5,059,764; 5,208,437; 5,265,114; 5,473,624;6,057,180; 6,172,325; 6,191,486; 6,239,406; 2002-0003130; and2002-0005396.

Other representative publications providing background on linkprocessing of memory circuits or similar laser processing applicationsinclude: “Laser Adjustment of Linear Monolithic Circuits,” Litwin andSmart, ICAELO, (1983); “Computer Simulation of Target Link Explosion InLaser Programmable Memory,” Scarfone, Chlipala (1986); “Precision LaserMicromachining,” Boogard, SPIE Vol. 611 (1986); “Laser Processing forApplication Specific Integrated Circuits (asics),” SPIE Vol. 774, Smart(1987); “Xenon Laser Repairs Liquid Crystal Displays,” Waters, Laser andOptronics, (1988); “Laser Beam Processing and Wafer Scale Integration,”Cohen (1988); “Optimization of Memory Redundancy Link Processing,” Sun,Harris, Swenson, Hutchens, Vol. SPIE 2636, (1995); “Analysis of LaserMetal Cut Energy Process Window,” Bernstein, Lee, Yang, Dahmas, IEEETrans. On Semicond. Manufact., Vol 13, No. 2. (2000).

Also, the following co-pending U.S. applications and issued patents areassigned to the assignee of the present invention and are herebyincorporated by reference in their entirety:

-   1. U.S. Pat. No. 5,300,756, entitled “Method and System for Severing    Integrated-Circuit Connection Paths by a Phase Plate Adjusted Laser    beam”;-   2. U.S. Pat. No. 6,144,118, entitled “High Speed Precision    Positioning Apparatus”;-   3. U.S. Pat. No. 6,181,728, entitled “Controlling Laser    Polarization”;-   4. U.S. Pat. No. 5,998,759, entitled “Laser Processing”;-   5. U.S. Pat. No. 6,281,471, entitled “Energy Efficient, Laser-Based    Method and System for Processing Target Material”;-   6. U.S. Pat. No. 6,340,806, entitled “Energy-Efficient Method and    System for Processing Target Material Using an Amplified,    Wavelength-Shifted Pulse Train”;-   7. U.S. Ser. No. 09/572,925, entitled “Method and System For    Precisely Positioning A Waist of A Material-Processing Laser Beam To    Process Microstructures Within A Laser-Processing Site”, filed 16    May 2000, and published as WO 0187534 A2, December, 2001;-   8. U.S. Pat. No. 6,300,590, entitled “Laser Processing”; and-   9. U.S. Pat. No. 6,339,604, entitled “Pulse Control in Laser    Systems.”

However, it is to be understood that this listing is not an admissionthat any of the above references are prior art under the Patent Statute.

The subject matter of the above referenced applications and patents isrelated to the present invention. References to the above patents andapplications are cited by reference number in the following sections.

SUMMARY OF THE INVENTION

An object of the present invention is to provide improved methods andsystems for processing a device, methods and systems for modeling sameand the device.

In carrying out the above object and other objects of the presentinvention, a method is provided for laser processing a multi-level,multi-material device including a substrate, a microstructure and amulti-layer stack, the stack having inner layers which separate themicrostructure from the substrate. The method includes the steps of: a)generating a pulsed laser beam having a predetermined wavelength andincluding at least one laser pulse having a predetermined characteristiccomprising at least one of a temporal shape and spatial shape; b)relatively positioning the microstructure and a waist of the laser beamin three-dimensional space based on at least a position measurementobtained at a reference location wherein the position measurement isused to obtain a prediction of a common location of the microstructureand the beam waist; and c) irradiating the microstructure with the atleast one laser pulse based on the predicted common location at a timewherein the beam waist and the microstructure substantially coincide,wherein the microstructure is cleanly removed with substantially maximumpulse energy density at the microstructure and wherein an undesirablechange to the inner layers of the stack and substrate is avoided.

The multi-level device may be a damascene semiconductor memory.

The predetermined characteristic may be a pulse rise time less than 2nanoseconds.

The rise time may be less than 1 nanosecond.

The predetermined characteristic may be a substantially square pulsehaving a duration of less than about 10 nanoseconds and a rise time lessthan about 2 nanoseconds.

The predetermined characteristic may be based on a model of laser pulseinteractions with materials of the multi-level, multi-material device.

The stack may have at least one outer dielectric layer covering themicrostructure, and the rise time may initiate cracking of the at leastone outer layer and avoids damage of the inner layers of the stack.

The inner layers may include at least 2 pairs of dielectric layers andmay further include at least 4 pairs of dielectric layers.

Pulse energy density at the substrate may be less than a damagethreshold of the substrate and the maximum pulse energy density at themicrostructure may be greater than the damage threshold of thesubstrate.

The predetermined characteristic may be 3D irradiance profile having abeam waist, Wy0, which approximates a narrow microstructure dimension.The irradiance profile may be characterized by a relative reduction inenergy density at the inner layers and the substrate caused by beamdivergence, whereby the energy density at the substrate is substantiallyreduced by at least the beam divergence and inner layer reflections.

The substrate may be a silicon substrate and the predeterminedwavelength may be about 1.064 μm.

The predetermined wavelength may be below an absorption edge of thesubstrate and pulse energy density at the substrate is reduced by atleast one of a plurality of predetermined factors of the modelincluding: (a) beam divergence; (b) stack surface reflection; (c) beamdiffraction; (d) stack multiple scattering; (e) internal stackreflection; (f) multi-layer interference; and (g) non-linear absorptionwithin the microstructure.

Peak pulse energy density at the substrate may be less than about 1/10of the maximum pulse energy density at the microstructure.

The dielectric layers may include silicon nitride and silicon dioxide.

The predetermined characteristic may be based on a physical property ofa material of the multi-material device.

The predetermined wavelength may be less than 1.2 μm and may be lessthan an absorption edge of the substrate.

The predetermined characteristic may comprise a non-circular spatialprofile based on a preselected numerical aperture and shape of a spot.The spot and the microstructure may be substantially correlated in atleast one dimension whereby a fraction of laser energy delivered to themicrostructure may be increased and irradiance of the stack and thesubstrate with the at least one laser pulse may be decreased.

A plurality of closely-spaced pulses may irradiate the microstructureduring the step of relatively positioning.

The pulses may have a duration in the range of a few picoseconds toseveral nanoseconds.

At least one of the pulses may be delayed by a delay line and spacingbetween the pulses may be in the range of about 20-30 nanoseconds.

Pulse energy of the pulses may be about 50% to 70% of pulse energy usedfor processing the microstructure with a single laser pulse.

The device may include a plurality of microstructures to be processed,and the method may further comprise repeating steps a), b) and c) untilall of the microstructures are processed.

Pulse energy density at the microstructure may be in the range of about0.1-5 μjoules over an area of less than about 10-20 square microns onthe microstructure.

The range may be about 0.1-3 μjoules.

The step of generating may include the step of shifting a firstwavelength of the laser beam to the predetermined wavelength. Pulseenergy is coupled into the microstructure more efficiently at thepredetermined wavelength than at the first wavelength while avoidingdamage to the inner layers and stack.

The predetermined wavelength may be within a spectral region whereinstack intensity reflection is substantially increased when compared toat least one other wavelength.

The stack intensity reflection may exceed about 60%, or may exceed about90%.

The predetermined wavelength may be less than the absorption edge of thesubstrate.

The method may further include: (1) obtaining information identifyingmicrostructures designated for removal; (2) measuring a first set ofreference locations to obtain three-dimensional reference data; (3)generating a trajectory based on at least the three-dimensionalreference data to predict a plurality of beam waist and microstructurelocations; and (4) updating the prediction during the step of relativelypositioning based on updated position information. The updated positioninformation may be obtained during the step of relatively positioning.

The updated position information obtained during the step of relativelypositioning may include data from a position encoder or from an opticalsensor.

The reference location may be an alignment target of the multi-materialdevice. The alignment target may be covered by at least one upper layerof the stack.

The predetermined characteristic may be irradiance profile at the beamwaist.

The irradiance profile may approximate a dimension of the microstructuresuch that a significant percent of laser energy is coupled to themicrostructure and background irradiance is reduced.

The dimension of the microstructure may be less than 1 μm.

The percent may be at least 60%.

The irradiance profile may be an elliptical Gaussian.

The irradiance profile may be a top hat along a length of themicrostructure and substantially Gaussian along a width of themicrostructure.

The irradiance profile may be substantially diffraction limited with anM-squared factor of less than about 1.1.

The microstructure may be a metal link.

The metal link may have a first dimension of less than 1 μm and thepredetermined characteristic may be a laser spot size less than about1.5 μm in the first dimension.

The substrate may be a silicon substrate and pulse energy density at thesubstrate may be less than about 1/100 the pulse energy density at themicrostructure.

Further in carrying out the above object and other objects of thepresent invention, a system is provided for laser processing amulti-level, multi-material device including a substrate, amicrostructure and a multi-layer stack, the stack having inner layerswhich separate the microstructure from the substrate. The systemincludes means for generating a pulsed laser beam having a predeterminedwavelength and includes at least one laser pulse having a predeterminedcharacteristic comprising at least one of a temporal shape and spatialshape. The system also includes means for relatively positioning themicrostructure and a waist of the laser beam in three-dimensional spacebased on at least a position measurement obtained at a referencelocation. The position measurement is used to obtain a prediction of acommon location of the microstructure and the beam waist. The systemfurther includes means for irradiating the microstructure with the atleast one laser pulse based on the predicted common location at a time:The beam waist and the microstructure substantially coincide, whereinthe microstructure is cleanly removed with substantially maximum pulseenergy density at the microstructure and wherein an undesirable changeto the inner layers of the stack and substrate is avoided.

A plurality of closely-spaced pulses may irradiate the microstructure bythe means for irradiating.

The means for generating may include means for shifting a firstwavelength of the laser beam to the predetermined wavelength. Pulseenergy is coupled into the microstructure more efficiently at thepredetermined wavelength than at the first wavelength while avoidingdamage to the inner layers and stack.

The system may further include means for obtaining informationidentifying microstructures designated for removal; (2) means formeasuring a first set of reference locations to obtain three-dimensionalreference data; (3) means for generating a trajectory based on at leastthe three-dimensional reference data to predict a plurality of beamwaist and microstructure locations; (4) means for updating theprediction based on updated position information.

The updated position information may include data from a positionencoder or from an optical sensor.

Still further in carrying out the above object and other objects of thepresent invention, a method is provided for laser processing amulti-level, multi-material device including a substrate, amicrostructure and a multi-layer stack. The stack has inner dielectriclayers which separate the microstructure from the substrate. The methodincludes generating a pulsed laser beam having a predeterminedwavelength and includes at least one laser pulse. At least reflectionsof the laser beam by the layers of the stack substantially reduce pulseenergy density at the substrate relative to at least one otherwavelength. The microstructure is processed with the at least one laserpulse. Pulse energy density at the microstructure is sufficient toremove the microstructure while avoiding damage to the substrate and theinner layers of the stack.

The predetermined wavelength may be less than an absorption edge of thesubstrate.

The predetermined wavelength may be in the range of about 0.4 μm-1.55μm, and may also be in the range of about 0.35 μm-1.55 μm.

The predetermined wavelength may be less than 1.2 μm.

The substrate may be a silicon substrate and the microstructure may be ametal microstructure.

The metal of the microstructure may include copper.

Pulse energy density at the substrate may be further reduced with apredetermined characteristic of the at least one pulse.

The predetermined characteristic may be based on a model of laser pulsematerial interaction.

The model may be a thermal model, or may be a multi-parameter model.

The step of generating may include the step of shifting the wavelengthof the laser beam from a first wavelength to the predeterminedwavelength. The predetermined wavelength may be based on materialcharacteristics comprising at least one of: (1) coupling characteristicsof the microstructure, (2) multi-layer interference, and (3) substratereflectivity.

The predetermined wavelength may be a shifted wavelength below anabsorption edge of the substrate.

Yet still further in carrying out the above object and other objects ofthe present invention, a system is provided for laser processing amulti-level, multi-material device including a substrate, amicrostructure and a multi-layer stack. The stack has inner dielectriclayers which separate the microstructure from the substrate. The systemincludes means for generating a pulsed laser beam having a predeterminedwavelength, and includes at least one laser pulse wherein at leastreflections of the laser beam by the layers of the stack substantiallyreduce pulse energy density at the substrate relative to at least oneother wavelength beyond the absorption edge. The system also includesmeans for processing the microstructure with the at least one laserpulse wherein pulse energy density at the microstructure is sufficientto remove the microstructure while avoiding damage to the substrate andthe inner layers of the stack.

The means for generating may include means for shifting the wavelengthof the laser beam from a first wavelength to the predeterminedwavelength. The predetermined wavelength may be based on materialcharacteristics comprising at least one of: (1) coupling characteristicsof the microstructure, (2) multi-layer interference, and (3) substratereflectivity.

Further in carrying out the above object and other objects of thepresent invention, a method for modeling interactions of a pulsed laserbeam including at least one laser pulse with a three-dimensional deviceincluding a substrate, a predetermined microstructure formed on thesubstrate and a plurality of other structures formed on the substrate isprovided. The method includes providing information regarding materialof the structures including the predetermined microstructure and thesubstrate. The method also includes determining optical propagationcharacteristics of at least a portion of the laser beam not absorbed bythe predetermined microstructure based on the information. The methodfurther includes determining at least one characteristic of the at leastone pulse based on the optical propagation characteristics.

The information may also regard spacing of the structures including thepredetermined microstructure and the substrate.

Still further in carrying out the above object and other objects of thepresent invention, a system for modeling interactions of a pulsed laserbeam including at least one laser pulse with a three-dimensional deviceincluding a substrate, a predetermined microstructure formed on thesubstrate and a plurality of other structures formed on the substrate isprovided. The system includes means for providing information regardingmaterial of the structures including the predetermined microstructureand the substrate. The system also includes means for determiningoptical propagation characteristics of at least a portion of the laserbeam not absorbed by the predetermined microstructure based on theinformation. The system further includes means for determining at leastone characteristic of the at least one pulse based on the opticalpropagation characteristics.

Yet still further in carrying out the above object and other objects ofthe present invention, a method for modeling interactions of a pulsedlaser beam including at least one laser pulse with a three-dimensionaldevice including a substrate, a predetermined microstructure formed onthe substrate and a plurality of other structures formed on thesubstrate is provided. The method includes providing informationregarding material and spacing of the structures including thepredetermined microstructure and the substrate. The method also includesdetermining optical propagation characteristics of at least a portion ofthe laser beam not absorbed by the predetermined microstructure based onthe information. The method further includes determining at least onecharacteristic of the at least one pulse which avoids undesirablechanges in electrical or physical characteristics of the substrate andthe other structures based on the optical propagation characteristics.

The interactions may be selected from a group comprising: reflectionsfrom at least one surface of the structures, internal reflections,polarization, interference effects, near field diffraction, scatteringand absorption.

The step of determining optical propagation characteristics may includecorrelating irradiance of a structure neighboring the predeterminedmicrostructure with a laser spot dimension.

Still further in carrying out the above object and other objects of thepresent invention, a system for modeling interactions of a pulsed laserbeam including at least one laser pulse with a three-dimensional deviceincluding a substrate, a predetermined microstructure formed on thesubstrate and a plurality of other structures formed on the substrate isprovided. The system includes means for providing information regardingmaterial and spacing of the structures including the predeterminedmicrostructure and the substrate. The system also includes means fordetermining optical propagation characteristics of at least a portion ofthe laser beam not absorbed by the predetermined microstructure based onthe information. The system further includes means for determining atleast one characteristic of the at least one pulse which avoidsundesirable changes in electrical or physical characteristics of thesubstrate and the other structures based on the optical propagationcharacteristics.

The means for determining optical propagation characteristics mayinclude means for correlating irradiance of a structure neighboring thepredetermined microstructure with a laser spot dimension.

Yet still further in carrying out the above object and other objects ofthe present invention, a method is provided for laser processing amultilevel, multi-material device including a substrate, amicrostructure and a multi-layer stack, the stack having inner layerswhich separate the microstructure from the substrate. The methodincludes generating a pulsed laser beam having a predeterminedwavelength and includes at least one laser pulse having a predeterminedcharacteristic wherein: a) the predetermined wavelength is below anabsorption edge of the substrate, and b) the at least one pulse hasduration less than about 10 nanoseconds, and a repetition rate of 10 KHzor higher. The method also includes relatively positioning themicrostructure and a waist of the laser beam in three-dimensional spacebased on at least a position measurement obtained at a referencelocation. The position measurement is used to obtain a prediction of acommon location of the microstructure and the beam waist. The methodfurther includes irradiating the microstructure with the at least onelaser pulse based on the predicted common location at a time wherein thebeam waist and the microstructure substantially coincide and themicrostructure is cleanly removed with substantially maximum pulseenergy density at the microstructure, and an undesirable change to theinner layers of the stack and substrate is avoided.

The predetermined characteristic may include a q-switched pulse shape.

The q-switched pulse shape may be generated with a microlaser.

The pulse duration of the at least one laser pulse may be in the rangeof 1-5 nanoseconds.

A plurality of pulses may be generated and delayed by a predetermineddelay based upon a physical property of a material of the multi-materialdevice.

The predetermined wavelength may be about 1.064 microns.

Yet still further in carrying out the above object and other objects ofthe present invention, a system is provided for laser processing amulti-level, multi-material device including a substrate, amicrostructure and a multi-layer stack, the stack having inner layerswhich separate the microstructure from the substrate. The systemincludes means for generating a pulsed laser beam having a predeterminedwavelength and includes at least one laser pulse having a predeterminedcharacteristic wherein: a) the predetermined wavelength is below anabsorption edge of the substrate, and b) wherein the at least one pulsehas duration less than about 10 nanoseconds, and a repetition rate of 10KHz or higher. The system also includes means for relatively positioningthe microstructure and a waist of the laser beam in three-dimensionalspace based on at least a position measurement obtained at a referencelocation. The position measurement is used to obtain a prediction of acommon location of the microstructure and the beam waist. The systemfurther includes means for irradiating the microstructure with the atleast one laser pulse based on the predicted common location at a timewherein the beam waist and the microstructure substantially coincide,the microstructure is cleanly removed with substantially maximum pulseenergy density at the microstructure, and an undesirable change to theinner layers of the stack and substrate is avoided.

Further in carrying out the above object and other objects of thepresent invention, a multi-level, multi-material device is provided. Thedevice includes a substrate, a microstructure, and a multi-layer stackhaving inner layers which separate the microstructure from thesubstrate. At least one of the inner layers has a predetermined physicalparameter based on interactions of a pulsed laser beam with the stack.The laser beam has a predetermined wavelength, wherein undesirablechanges to the substrate and the inner layers of the stack are avoidedduring laser processing with the pulsed laser beam.

The predetermined physical parameter may include at least one of layerthickness, layer material, and index of refraction.

The stack may include a quarter-wave stack of dielectric layers and mayalso include a precision spacer layer located between layers of thestack.

The above object and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a block diagram of a laser system which generates a laserpulse in response to a trigger signal obtained from a control system,the pulse having a temporal shape including a fast rise and fall time,and a duration selected for the material processing application of thepresent invention;

FIGS. 1 b and 1 c are views partially broken away illustrating amulti-layer, multi-material device wherein a laser pulse withpre-determined temporal and spatial characteristics irradiates thedevice; FIG. 1 b is a first side sectional view of a portion of thedevice, showing a target structure having a rectangular cross-section,wherein a high numerical aperture laser beam, having a non-unity aspectratio, is incident on the target structure having a plurality of layersforming a stack; FIG. 1 c is a second side sectional view of a portionof the device, orthogonal to the first, showing a rectangular targetstructure, wherein a high numerical aperture laser beam, having anon-unity aspect ratio, is incident on the target structure;

FIG. 2 a is a block diagram of a prior art laser system which shows aconventional q-switched or Gaussian pulse;

FIG. 2 b is a view of a conventional multi-layer structure having asingle oxide layer between the link and substrate, therefore beinglocated in proximity to a substrate, with a conventional q-switchedlaser pulse irradiating and substantially overfilling the narrowdimension of the target structure;

FIG. 3 is a graph of reflection as a function of wavelength of amulti-layer stack having 28 layers in 14 pairs, the stack representativeof a device processed with a method and system of the present invention;

FIGS. 4 a and 4 b are top views and associated graphs which illustratethe effect of irradiating the target structure with laser beam profilesof varying dimension with respect to the target structure; FIGS. 4 a and4 b show the result of truncating a representative non-uniform Gaussianshaped laser spatial profile, wherein the energy enclosed by the targetstructure is strongly affected, the energy at the target edge varies,and potential stray radiation effects result from energy not absorbed bythe target structure;

FIG. 4 c is a side schematic view of a plurality of microstructuresformed on a layer and which illustrate that for decreasing spacing(pitch) inter-reflections and stray energy result in irradiation ofneighboring target structures;

FIGS. 5 a and 5 b are graphs which show the reduction in irradiance onthe device as a function of depth resulting from precise positioncontrol of a high numerical aperture beam (at the top surface), whereinthe position and depth of focus of the beam provides for processing ofthe target structure without creating undesirable changes to othermaterials; In particular, FIG. 5 a illustrates the increase in spot areawith for various spherical and elliptical Gaussian irradiancedistributions, for a representative multi-layer stack used in a coppermemory process;

FIG. 5 b normalizes the defocus function relative to the energy density(fluence) at the target location;

FIGS. 6 a and 6 b are schematic views of a stack of layers formed on awafer substrate and which illustrate exemplary results obtained with aray trace simulation used to estimate the level of radiation impingingon the internal layers and adjacent links with a specified beamnumerical aperture;

FIGS. 7 a, 7 b, 8 and 9 are views of images taken from detectors andwhich illustrate, on a continuous scale spanning 5 decades, simulatedpatterns of radiation at the surface, substrate, and with the stackremoved respectively;

FIG. 10 is a schematic diagram of a system for measuring fiducials orother alignment targets;

FIG. 11 is a graph of reflectivity versus outer layer thickness;

FIG. 12 shows a pair of graphs of reflectivity versus thickness of theouter oxide layer for two different laser beam wavelengths;

FIG. 13 is a schematic diagram of a system for automatically controllingpulse energy based on a thickness measurement;

FIG. 14 a shows schematic and graphical representations of an effect ofdebris on signal fidelity during alignment measurements;

FIG. 14 b shows similar representations with improved signal fidelityafter cleaning with a pulsed laser beam;

FIGS. 15 a-15 c show various arrangements for combining laser pulses orgenerating a sequence of closely spaced pulses using optical orelectronic delay methods; FIG. 15 a illustrates use of multiple laserswith delayed triggering; FIG. 15 b illustrates a basic arrangement witha single laser and an optical delay path; and FIG. 15 c illustrates yetanother modular optical delay line providing for pointing stability andsimplified alignment;

FIG. 16 is a graph of temperature versus time which illustratessimulation results for metal link (top) and substrate (bottom)irradiance with a pair of delayed pulses wherein the substratetemperature decays rapidly exhibiting a differential thermal property ofthe materials; the two laser pulses each had a square temporal shape;

FIG. 17 is a series of schematic views of a metal link which illustratea multiple pulse sequence wherein: (1) a first pulse irradiates themetal link; (2) debris is left after removing the link; (3) a secondpulse with a spatial pulse shape is used wherein the central zone isattenuated, the second pulse having a lower peak energy density than thefirst pulse; and (4) 25 ns after the start of the first pulse the debrisis removed;

FIG. 18 is a block diagram of a system which generates and controllablyselects pulses;

FIG. 19 is a block diagram of a system of the present invention whereina portion of a high repetition rate pulse train (e.g., 1 μHz) isselected and a high speed beam deflector (e.g., electro-optic oracousto-optic device) synchronized with microstructure positions is usedto process a single microstructure with multiple pulses during relativemotion; and

FIG. 20 is a block diagram of another system of the present inventionwherein a beam deflector is used to spatially split a single pulse so asto irradiate either one or two microstructures (or none) with a pair ofpulses during relative motion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One aspect of the invention is removal of a microscopic target structurewhich is part of a multilayer, multimaterial device, wherein laserenergy is incident on several materials having dissimilar optical andthermal properties. One application is memory repair. A new fabricationprocess (Damascene) includes a copper target structure, multipledielectric layers in the form of a “stack,” and functional circuitrydisposed at the dielectric layers. The target structure and layers aretypically formed on a silicon substrate. This is illustrated in FIGS. 1b and 1 c and corresponds to a device processed with an embodiment ofthe present invention. This will be referred to as a “multilevel”process.

With the use of more complex structures at finer scale (e.g., at orbelow a wavelength of visible light), considerations for reliableoperation of laser processing system increase to meet the standards forhigh yield in the semiconductor industry.

Aspects of the invention include methods and subsystems for operation ofthe laser processing system. At the microscopic scale, the laser beamwaist diverges rapidly due to the small spot size and depth of focus.The materials within the 3D beam location may include functionalcircuitry. In an automatic system, robust measurement of targetlocations is used in conjunction with database information to position alaser beam in three dimensions at high speed. The interaction of a laserbeam within the multilevel device influences yield. Modeling of thermalinteraction is useful of understanding and predicting performance in thethermal processing regime. However, at the microscopic scale, a moredetailed understanding of interaction based on physical optics is alsobeneficial.

In the following sections, detailed aspects of spatial and temporalpulse shaping, three-dimensional measurement and prediction, devicemodeling and process design are disclosed with emphasis on solving theproblem of cleanly removing links on a multilevel device, wherein damageis avoided to inner layers and functional circuitry between a link andthe substrate. However, various methods, subsystems, and experimentalresults may also be applied for link processing of conventional singleinner layer devices, and generally for processing microstructuressurrounded by materials having dissimilar thermal or optical properties.

Processing Links on a Multilevel Device

A pulsed laser beam, the beam having pre-determined characteristics forprocessing of microscopic structures, is used to cleanly remove at leasta portion of a target structure. An application of the method and systemof the present invention is severing of highly reflective copper linkswhich are part of a high speed semiconductor memory device. The methodand system of the present invention is particularly advantageous forprocessing of targets having a sub-micron dimension, including targetswith a dimension below the wavelength of the laser beam. The target isseparated from a semiconductor substrate by a multi-layer stack, whichmay have several dielectric layers. Furthermore, both the temporal andspatial characteristics of the pulse may be selected or controlled basedon the thermal and optical properties of the microscopic target,underlying layer materials, and the three-dimensional layout of thedevice structure, including the spacing of target structures andfunctional inner conductor layers.

FIGS. 1 a-1 c generally show an embodiment of the present invention Alaser pulse 3 irradiates a rectangular target structure ormicrostructure 10, side views of which are shown in FIGS. 1 b and 1 c,with a focused beam. In a preferred embodiment, an output from shortpulse amplified laser system 1 is generated to produce the pulse 3 whichhas a rise time 4 fast enough to efficiently couple energy into a highlyreflective target structure. The duration 5 is sufficient to process thetarget structure wherein at least a portion of the structure is cleanlyremoved without leaving residue, slag, or other debris. The fall time 6is preferably fast enough to avoid creating undesirable damage to thelayers or substrate.

The temporal pulse shape is selected, in part, based on physicalproperties of the target microstructure 10, for instance, thickness,optical absorption, thermal conductivity, or a combination thereof. Inan advantageous embodiment of the invention, the processing will occurwith a single pulse having a fast edge leading relative to a selectedpulse duration of several nanoseconds. In an alternative embodiment, thelaser output may be a series of narrow q-switched or rectangular pulses,with very fast rise time, for example 800 ps pulses representative ofthe output of commercially available q-switch micro-lasers. The pulsesmay be delayed with respect to each other so as to provide a burst ofpulses to irradiate the target structure. The laser output may begenerated with a combination of a high bandwidth seed laser diode andfiber optic amplifier with Raman shifting, or with a waveguide amplifiersystem. Alternatively, a desirable pulse characteristic may be providedwith various modified q-switched systems or with the use of high speedelectro-optic modulators. Other pulse shapes may be selected for thematerial processing requirements. For instance, a sequence of closelyspaced pulses having duration from a few picoseconds to severalnanoseconds is taught in Reference 5.

In one embodiment, a high bandwidth MOPA configuration is used toamplify the laser output of a high speed semiconductor diode. Generationof various pulse shapes and duration with direct modulation of the diodeis considered advantageous, provided any affect associated with variableamplitude drive waveforms does not affect overall performance. Furtherdetails of various aspects of pulse generation and amplification can befound in references 5 and 6 (e.g., in '471—Reference 5—FIGS. 5 andcolumns 14-16).

As indicated above, embodiments of the laser system may include fiberoptic amplifiers which amplify the preferred square pulse shapegenerated by a seed laser. The seed laser may be a high speedsemiconductor diode or the shaped output of a modified q-switchedsystem. The amplified output may be matched in wavelength to the inputor Raman-shifted as taught in References 4 and 6 (e.g., in Reference 6,FIGS. 12-13 and column 14, line 57—column 19, line 3). Wavelengthshifting of a short pulse q-switched laser output is generally taught in'759 Reference 4.

In an alternative arrangement the seed laser is a semiconductor diodeand the optical amplifier is a waveguide amplifier. Advantages of anembodiment with a waveguide amplifier when compared to a fiber systeminclude avoidance of Raman shifting, lower pulse distortion at the speedof operation, and, with proper design, minimal thermal lensing. Aprecision anamorphic optic system is used to optimize coupling betweenthe seed and amplifier. Basic description of waveguide amplitude andlasers can be found in product literature provided by Maxios, Inc. andin the article “CW and passively Q-switched Cladding Pumped PlanarWaveguide Lasers,” Beach et. al. Yet another amplifier system includinga 28 DB planar waveguide amplifier for use at 1.064 μm wavelengths wasdeveloped by University of Southhampton and described in “A DiodePumped, High Gain, Planar Waveguide, Nd:Y3A15O12 Amplifier.”

In an alternative arrangement, for generation of a fast rising pulse orother desirable shape, a plurality of q-switched micro-lasers can beused. The modules produce a q-switched waveform with pulse durations ofabout 1 nanosecond or less, for example 800 ps to 2 ns for commerciallyavailable units. An example of a commercially available laser is theAOT-YVO-1Q available from Advanced Optical Technology (AOTLasers.com).These recently developed short pulse, active q-switch lasers can betriggered with a TTL pulse at a variable repetition rate whilemaintaining specified sub-nanosecond timing jitter. In general, thepulse shape incident on the target microstructure will varysignificantly at repetition rates approaching the maximum rate.Reference 9 teaches methods of maintaining a constant pulse shapedespite variations in the temporal spacing of pulses incident on atarget (e.g., see the figures and associated specification). AOT offersa pulsewidth of 2 nanoseconds available at a repetition rate of 20 KHz.Frequency doubled versions are also available (532 nm). IMRA Americareports 800 ps pulses with the PicoLite system, and high peak power wasobtained with fiber amplification at repetition rates up to 10 KHz.Shorter pulsewidths, for instance about 1 ns or less, are available atslower repetition rates.

As known in the art and illustrated in Reference 5 (e.g., FIGS. 1 c, 2),the q-switched waveforms may approximate (at least to 1st order) asymmetric Gaussian shape, or a fast rising pulse with an exponentialtail, depending on the stored energy. With reference to FIGS. 15 a-15 c,a series of devices, with appropriate delays introduced by a pluralityof triggering signals, or delays of a trigger signal with a delay line,is used to generate a series of spaced apart pulses. The optical outputsare preferably combined with appropriate bulk optics (polarizationsensitive), fiber optics, or waveguides to form a single output beam.The resultant addition of the q-switched waveforms produces a fast risetime characteristic and relatively short duration. An optical amplifier122 may be used to increase the output power as needed.

FIG. 15 a shows a schematic of one basic embodiment with bulk optics,where a beam combiner 123 is used to deliver the output of two lasers120,121 to an amplifier 122. A delay circuit 126, which may beprogrammable, controls triggering. Polarization optics 127,128 are usedto provide the proper input to the beam combiner. In one arrangement thepulses are spaced apart and appear as a high frequency burst 124. In asecond arrangement triggering of the second pulse occurs at a slightlydelayed (but controlled) position which produces a characteristicapproximating a square pulse shape 125. In the latter arrangement thecontrolled delay is about 50% of the FWHM. Those skilled in the art willrecognize that alternative arrangements may be used with multipleamplifiers, combiners, with bulk, fiber, or integrated opticarrangements.

Generation of multiple pulse waveforms may also include some form ofactive q-switching of two separate microlasers or detecting a firstpulse from a passively q-switched laser and subsequently triggering anactively q-switched laser or MOPA relative to the first pulse.

FIG. 15 b is a basic schematic showing the use of a single laser 140wherein the laser output is divided by beam splitter 142, whereby aportion of the beam propagates along a path 141, followed by combiningwith combiner 143, after polarization adjustment with rotator 146 whichmay be a half-wave plate. An optional optical amplifier 145 may then beused to produce higher output power.

In an arrangement using a single laser and an optical delay line, theoptical system will preferably be stable and easy to align. FIG. 15 cshows an exemplary embodiment wherein the use of opposing corner cuberetroreflectors 130 makes the setup insensitive to tilt of the foldingelements. The angular alignment of the delayed beam paths 131,132 isvery stable even in a high vibration environment. One of the cornercubes in each pair of retroreflectors 130 is initially adjusted in theX/Y translation and Z rotation to get the transverse position of thedelayed beam path centered. Each of the λ/2 retarders 133 in the mainbeam path is adjusted so that vertical or horizontally polarized lightwill have its polarization rotated by 45 degrees. The λ/2 retarder 133in the second delay loop is adjusted so that vertical or horizontalpolarized light will have the polarization rotated by 90 degrees causingthe delayed pulse in the second loop to circulate twice before exiting.The peak-to-peak spacing of the output waveform 135 (e.g., 4 combinedpulses) is controlled by the length of the delay loops. If non-equalamplitudes for the delayed pulses are desired, the λ/2 retarders 133 inthe main beam can be set for a polarization of other than 45 degrees.Likewise, the pulse shape can be varied at the time a system is setup orpossibly in operation by manually or automatically controlling thespacing. Those skilled in the art of laser pulse generation and shapingwill appreciate the advantages of the compact and modular arrangementfor short pulse for typical delays ranging from a few nanoseconds totens of nanoseconds. For instance, U.S. Pat. No. 5,293,389 to Hitachidescribes a polarization-based fiber delay line for generating laserpulses of decreasing amplitude for generating longer pulses, forinstance 100 ns or longer.

Another means of producing a shaped pulse is to use the modulatorapproach to chop the leading edge or tail of the pulse but with atwo-stage or shaped modulation voltage pulse. For example: with a 10 nsq-switched pulse, the modulator could have 100% transmission for thefirst 1-5 ns followed by 25% transmission for the remainder of thepulse. Early pioneering work by Koechner (U.S. Pat. No. 3,747,019) andSmart (U.S. Pat. No. 4,483,005) demonstrate exemplary amplitude andpulse shape control methods using electro-optic modulators.

The multiple pulses shown in FIGS. 15 a-15 c may or may not have thesame wavelength, and the temporal shape of a pulse may be varieddepending upon specific requirements. For example, in certainembodiments an output may be a q-switched pulse of short duration andhigh peak power combined with a lower power square pulse shape.

Referring to FIGS. 1 a and 1 b, during system operation for memoryrepair, position information, obtained with a precision measurementsystem, is used to relatively position the focused beam waist of thepulsed laser at a location in space 7,8,9 to substantially coincide withthe target 10 three-dimensional coordinates (Xlink,Ylink,Zlink). Atrigger pulse 2, generated at a time where the laser beam waist andtarget position substantially coincide, operates in conjunction with thelaser and associated control circuitry in laser subsystem 1 to producean output pulse.

References 2 and 7 describe details of a method and system for precisionpositioning, including three-dimensional beam waist positioning.Reference 7 describes a preferred embodiment for producing anapproximate diffraction limited spot size with a range of spot sizeadjustment (e.g., FIGS. 7-9 of WO0187534 ('534) and the associatedspecification), and a preferred method and system for three-dimensionalpositioning of the beam waist. Three-dimensional (height) information isobtained, for instance with focus detection, and used to estimate asurface and generate a trajectory (e.g., FIGS. 2-5 of '534 and theassociated specification). The laser is pulsed at a locationsubstantially corresponding to the three-dimensional position of thelink (Xlink, Ylink, Zlink) (e.g., FIGS. 10 a-b of '534 and theassociated specification).

In practice, the three-dimensional measurement and positioning are usedto compensate for topographical variations over a wafer surface, orother position variations introduced in a system (mis-alignment). Thesevariations are generally system or application dependent and may exceedseveral microns, which in turn exceeds the depth of focus of the focusedlaser beam. In some micro-machining applications the system positioningrequirements may be relaxed if certain tolerances are maintained, or ifexternal hardware manipulates the device position, as might be done witha micro-positioning sub-system. The device may comprise a miniature part(e.g., single die) which is positioned by an external micro-positioningsubsystem to a predetermined reference location. Similarly, if aminiature part has a pre-determined tolerance the positioning may bebased on single measurement at a reference location or perhaps a singledepth measurement combined with a lateral (X,Y) measurement. Forprocessing of multilevel devices on wafers, (e.g.: 300 mm) at high speedit is expected that densely sampled three-dimensional information willimprove performance, particularly as link dimensions shrink.

In applications requiring very high speed operation over a large surface(e.g., 300 mm wafer), an alternative method is to combine informationwhich may be predetermined (e.g., the plane of a wafer chuck relative toa beam positioner plane of motion measured during a calibration process)with dimensional information obtained from each part to be processed.For example, in '534, FIGS. 1-2, a fraction of the tilt of region 28 maybe associated with fixturing). For example, the steps may include (a)obtaining information identifying microstructures designated forremoval, (b) measuring a first set of reference locations to obtainthree-dimensional reference data, (c) generating a trajectory based onat least the three-dimensional reference data to obtain a prediction ofbeam waist and microstructure surface locations, (d) updating theprediction during relative motion based on updated position information,the updated position information obtained from a position sensor (e.g.,encoder) and/or from data acquired during the relative motion. Theadditional data may be measurement data acquired at additional alignmenttarget or at other locations suitable for an optical measurement (e.g.,dynamic focus). Reference 2 describes a system wherein a precision waferstage is used to position a wafer at high speed. A method of obtainingfeedback information with resolution of a fraction of one nanometer isdisclosed wherein interferometric encoders are used, and such a highprecision method is preferred. In Reference 2 it was noted that otherconventional laser interferometers may also be used. FIGS. 9-11 andcolumns 5-6 of Reference 2 describe aspects of the precision measurementsubsystem associated with the precision positioning apparatus.Additionally, designated reference locations on the workpiece (e.g.,wafer) which may be an x,y alignment target or a region suited for athree-dimensional measurement may be used for various applications. Itshould also be noted that height accuracy of about 0.1 μm was reportedin “In-situ height correction for laser scanning of semiconductorwafers,” Nikoonhad et al., Optical Engineering, Vol. 34, No. 10, October1995, wherein an optical position sensor obtained area averaged heightdata at high speeds. Similarly, a dynamic focus sensor (e.g., astigmaticsystems used for optical disk tracking and control) may be used toobtain height information provided the data rate is fast enough tosupport “on the fly” measurement.

Various combinations of the above technologies can be used dependingupon the application requirements. A combination may be based on thenumber and typical distribution over a device of microstructuresdesignated for removal. When a large number of repair sites aredistributed across a device, the throughput may be maximized byproviding updates “on the fly.”

In an application of the invention, the target structure 10 is providedas a part of a multi-material, multi-layer structure (e.g., redundantmemory device). The multi-layer stack having dielectric layers 14,15provides spacing between the link and an underlying substrate 17. In onetype of multi-layer memory device, alternating layers of Silicon Dioxide15 and Silicon Nitride 14 may be disposed between a copper link targetstructure 10 and a Silicon substrate 17. The copper target structure isgenerally located in proximity to other similar structures to form a 1-Dor 2-D array of fuses which are designated for removal. In addition tothe copper link structure, underlying conductors 16 disposed as part ofthe functional device circuitry, may be in proximity to the linkstructure, and arranged in a series of patterns covered by relativelythin (<0.1 μm typical) Silicon Nitride 14 and thicker (˜1 μm typical)Silicon Dioxide 15 materials.

The irradiance distribution at the link may substantially conform to adiffraction limited, circular Gaussian profile. In another usefulembodiment, the beam has an approximate elliptical Gaussian irradianceprofile, as might be produced with an anamorphic optical system, or witha non-circular laser output beam. In one embodiment, the incident beamhas a non-uniform aspect ratio 12:11 as also illustrated in FIG. 4 b(e.g., 3:1). Alternatively, rectangular or another chosen spatialprofiles may be implemented in a lateral dimension. For example,Reference 1 discloses various advantageous methods and optical systemsfor “non-Gaussian” spatially shaping of laser beams for application tomemory repair.

With the nearly diffraction limited elliptical Gaussian case, thepreferable minimum beam waist dimension at location 11 approximates thenarrow target 10 dimension of FIG. 1 b, which, in turn, produces highpulse energy density at the link. Further, with this approach, a highfraction of the laser energy is coupled to the link and backgroundirradiance is reduced.

A typical copper link used in a present memory has width and thicknessof about 1 μm or less, for example, 0.6 μm, and length of about fivemicrons. Future memory requirements are expected to further reduce thescale of target dimensions. The minimum beam waist dimension Wyo at 11will typically overfill the sub-micron link to some degree, whereasaspect ratio Wxo/Wyo 12,11 with Wxo a few microns along the link, canfacilitate clean link removal. Additionally, rapidly decreasing energydensity on the layers 14,15 and substrate 17 is achieved through defocusof the high numerical aperture beam portion 11.

The graphs of FIGS. 5 a and 5 b illustrate the estimated defocus forvarious aspect ratios, relative to a circular Gaussian and an ellipticalbeam at best focus. FIG. 5 a shows the very rapid falloff of a 1.6 μmcircular Gaussian (0.002 mm numerical divisions =2 μum). FIG. 5 b showsa normalized result to scale the energy density at best focus for thedifferent spot shapes. These results indicate that with precision beampositioning in depth, wherein the power density is maximized at thetarget site, at relative reduction in energy density of more than onedecade occurs at the substrate level for an exemplary multi-layer stackused in a copper based process for memory fabrication. Further, therapid defocus relative to the waist Wy0 is beneficial for avoiding innerlayer damage, provided the “tails” of the incident beam irradiatefunctional inner layer 16 (e.g., copper) at a low level.

In one embodiment for processing a multilevel device, copper linkremoval is initiated with application of the fast rise time pulse,having a nominal 10-90% rise time 4 in a preferred range of less than 1nanosecond to about 2 nanoseconds. A pulse duration 5 in the range ofabout 2 nanoseconds to 10 nanoseconds is preferable to sever the linkwhile limiting thermal diffusion. Pulse energies in the range of about0.1 microjoules (μj) to 3 μj were effective, with a preferred typicalrange of about 0.1-5 μj considered sufficient margin for spot shape andprocess variations. The preferred pulse duration may be selected basedupon the nominal link thickness specifications, or based on a model ofthe dissimilar thermal and optical properties of adjacent materials.During the pulse duration, thermal shock of top layer 13 and thermalexpansion of the target 10 result in explosion of the link throughruptured top oxide layers 13, which in turn reduces the stress at thelower corner of the link structure adjacent to the layer 14. The laserpulse is rapidly terminated, preferably within a few nanosecond falltime 6 after the explosion, at a time just after the thin link iscleanly severed, and prior to a time the lower corner of the linkresults in cracking of at least layer 14. Further details and resultsrelated to the interaction of a laser pulse with a metal link andoverlying layers is disclosed in references 4 and 5. The '471 patent andthe associated specification describe the interaction process (e.g.,FIGS. 1 a, 1 b, 11 a, 11 b, and in column 18).

Hence, a combination of the spatial characteristics (e.g., beam waistshape and position) and the temporal (e.g., rise time 4, flatness, andduration 5) pulse characteristics avoids undesirable cracking of lowerlayers 14,15, avoids significant pulse interaction with inner layerconductor 16, and limits substrate 17 heating. Hence, despite the highreflectivity of the copper link at visible and near infraredwavelengths, and the expectation in the prior art of incomplete removaland damage to surrounding structures and substrate, the target structureis processed without undesirable damage to other structures. It is alsoknown that copper, in addition to having nearly maximum reflectance inthe near IR, is also more reflective than other link materials (e.g.,aluminum, platinum). Nevertheless, due to the optical interaction of thenear IR beam with the target and the optical and thermal properties ofadjacent (overlying) layers, the preferred copper material can beprocessed.

Furthermore, near IR (Infrared) wavelengths also conveniently correspondto wavelengths where high bandwidth laser diodes are available, and tothe spectral range where optical amplification of the pulsed laser beamcan be efficiently produced with fiber and waveguide amplifiers. Thoseskilled in the art will recognize that amplified laser diode outputs,having a desired temporal pulse shape, may also be frequency multipliedto produce visible laser outputs when advantageous. The fast rise timeof semiconductor diodes is particularly advantageous for producing afast rise time, square pulse characteristic. Future developments invisible diode and optical amplifier technology may support direct pulseamplification in the visible range.

In a preferred system for copper link blowing, the link width is afraction of one micron and the link spacing (pitch) is a few micronswith present process technology. The link width may typically correspondto a wavelength of visible light. Further, at the microscopic scale ofoperation, where the lateral and/or thickness dimensions of thematerials of FIGS. 1 b and 1 c are on the order of the laser wavelength,the thickness and indices of refraction of the stack materials cansignificantly affect the overall optical characteristics of the stack.

In one embodiment of the invention, a preferred reduced wavelength isselected in the visible or near infrared range wherein a non-absorptiveoptical property of the layers (e.g., interference or reflection loss)is exploited. The device structure of FIGS. 1 a and 1 b can be damagedwith substantial absorption within the lower layers, such damage isprohibitive because of the presence of adjacent circuitry. This is incontrast to link processing with the prior art system of FIG. 2 b whereinner layer damage is not generally detrimental to overall deviceperformance.

U.S. Pat. No. 6,300,690 (Reference 8) describes a system and method forvaporizing a target structure on a substrate. The method includesproviding a laser system configured to produce a laser output at thewavelength below an absorption edge of the substrate. Furthermore,Reference 4 discloses benefits of a wavelength less than 1.2 um forprocessing links on memory devices wherein the substrate is Silicon,namely smaller spot size and shorter laser pulsewidths. In accordancewith the present invention, improved performance can be realized byexploiting the non-absorbing stack properties with wavelength selection.Furthermore, at least one of precision positioning of a high numericalaperture beam, spatial shaping of the spot, or temporal pulse shapingalso will provide for reduced energy at the substrate. The resultcorresponds to a relatively low value of energy expected to be depositedin the substrate, despite an incident beam energy necessary to depositunit energy in the target structure sufficient to vaporize the targetstructure.

The factors affecting the energy deposited in the substrate are, ineffect, multiplicative. Likewise, at short visible wavelengths, copperis absorbing (e.g., about 50% at 500 nm, 70% in the near UV, compared to2% at 1.064 um) so less energy is required for clean removal, at leastan order of magnitude. The preferred identified wavelength correspondingto a relatively low value of the energy expected to be deposited in thesubstrate is within a visible of near IR region of the spectrum. Amodel-based approach may be used to estimate the shortest wavelengthwith sufficient margin for a specified dielectric stack, spot position,tolerance, temporal and three-dimensional spatial pulse characteristics.

For processing on links on multilevel devices with Silicon substrates,the limiting wavelength corresponding to a relatively low value of theenergy expected to be deposited in the substrate (e.g., below the imagethreshold) may be within the green or near UV region of spectrum, butthe use may require tightly controlled system parameters, includingpossible control of the stack layer thickness or index of refraction.

With wavelength selection in accordance with the present invention,where the internal transmission and preferably reflection of the stackis at or near a maximum, stack layer damage is avoided. Furthermore,decreasing substrate irradiance, while simultaneously providing areduced spot size for link removal (at or near diffraction limit), ispreferred provided irradiation of functional internal layers is withinacceptable limits. Spectral transmission curves for typical largebandgap dielectric materials generally show that the transmissiondecreases somewhat at UV wavelengths. For example, in HANDBOOK OF LASERSCIENCE AND TECHNOLOGY, the transmission range of Silicon Dioxide isspecified as wavelengths greater than 0.15 μm. The absorptioncoefficient of both Silicon Nitride and Silicon Dioxide remainsrelatively low in the visible range (>400 nm) and gradually increases inthe UV range.

FIG. 3 is a graph which illustrates the estimated back reflectionproduced by a representative multi-layer stack of 14 Silicon Dioxide 15and Silicon Nitride 14 pairs over a range of near IR wavelengths, wherethe thickness of the layers is about 1 μm and 0.07 microns,respectively. In accordance with the present invention, a large numberof layers can be accommodated, and may range from about 4-28 dependentupon the process (e.g., sometimes multiple layers may separate afunctional conductor layer).

By way of example, it is shown that significant reflection occurs overrelatively broad wavelength range. A single layer disposed as aninternal layer 14 will typically reflect roughly 2% at each surface overthe visible and near IR spectrum. It is well known in the art of linkand semiconductor processing that Silicon absorption varies by orders ofmagnitude in the near IR spectral range. Further, studies of Siliconmaterial processing have shown that the absorption is unstable andnon-linear with increased laser power and substrate heating atwavelengths near the absorption edge, as taught in reference 4. However,as stated above, the shorter wavelengths are preferred to producesmaller spots (references 4-6, and 8) and higher energy concentration atthe link position.

In accordance with the present invention, exploiting the layerreflection with wavelength can further enhance the system performanceand supplement the benefits associated with temporal and spatial controlof the pulse in a preferred short wavelength range. Such wavelengthselection is regarded as particularly advantageous at wavelengths wherethe substrate absorption would otherwise greatly increase, andsignificant margin can be obtained when the number of layers 14,15disposed between the link and substrate substantially exceeds the numberof overlying layers 13. A preferred structure for processing willcomprise a substantial number of layers, with large reflectance at apredetermined short wavelength, the wavelength being well matched forgeneration of the preferred fast square temporal pulse shape.

Standard laser wavelengths in the range of FIG. 3 include 1.047 μm and1.064 μm, the latter being a standard wavelength of semiconductordiodes. Further, custom wavelengths include 1.08 μm, and otherwavelengths generated with Raman shifting. Those skilled in the art willrecognize that frequency multiplication of the near IR wavelengths canbe used to generate short wavelengths, and with appropriate designmultiple wavelengths may be provided in a single system. For instance apreferred temporal pulse shape, with a fast rise time, may be generatedin the green portion of the visible spectrum by frequency doubling anear IR laser.

In an alternative embodiment, wavelength tuning is used to match thewavelength to the approximate peak reflectance of the stack. Such anarrangement may be particularly advantageous for adjustment of a laserwavelength at the edge of the reflectance range (i.e., “cutoff” range)over a limited wavelength range, whereby sensitivity to tolerances inthe material thickness and index of refraction are avoided. As notedabove, further discussion of laser amplifier systems and application toother link structures can be found in references 4-6.

Generation of the pulsed laser beam may include the step of shifting thewavelength of the laser beam from a first wavelength to a predeterminedwavelength. The predetermined wavelength may be based on materialcharacteristics comprising at least one of: (1) coupling characteristicsof the microstructure, (2) multi-layer interference, and (3) substratereflectivity.

Experimental results have shown that at a wavelength of 1.047 μm, wherethe absorption of Silicon in orders of magnitude higher than at 1.2 μm,substrate damage is avoided with a short q-switched (standard) pulse andthe stack characteristic of FIG. 3. However, the results with a standardlaser having a q-switched temporal pulse shape showed cracking of anoxide layer 14 below the link. The relatively slow rising q-switchedpulse shape, which for a Gaussian approximation is a substantialfraction of the duration, was considered a limiting factor for linkremoval without cracking of the inner layer based on experimentalresults. However, based on the teachings of the prior art, severe damageto the Silicon substrate would be expected at the 1.047 μm wavelengthbecause the absorption is orders of magnitude higher than at awavelength corresponding to maximum transmission. In accordance with theteachings of the present invention, the spatial pulse characteristicsand the stack reflection are important factors to consider so as toavoid inner layer and substrate damage and short wavelengths ofoperation (which also provide for a smaller spot size and higher energyconcentration at the link). Further, in accordance with the presentinvention, a predetermined square pulse shape generated at a laserwavelength of 1.047 μm would be expected to produce clean removalwithout undesirable changes to the stack and substrate.

Laser Processing and Process Design at the Sub-Micron Scale

Furthermore, in an exemplary advantageous embodiment for shortwavelength processing of reflective microscopic structures, aspecification for a multi-layer stack may be considered in processdesign. For example, a quarter-wave stack of alternating dielectrics orother suitable arrangement having a large difference in the index ofrefraction, and high transmission within each layer, is specified at aselected wavelength. It can be shown that very high reflectance isachievable, the quarter-wave stack being easily computed in closed formand modeled. Hence, the method and system of the present invention canbe used effectively with other aspects of process design, and may beadvantageous where the absorption of deeply buried layers and thesubstrate is relatively high, or where the width of a target structureis well below the laser wavelength.

The design of the device structure may have certain constraints relatedto the layout of the circuitry. As such, certain thickness and materialfor a certain layer may be defined, for instance an insulator in a planeof a conductor having the approximate thickness of the conductor, orrelated to the thickness of the conductor. It may be possible to selecta material having a different index of refraction than the specifiedlayer. A specified thickness may be based on the estimated reflection atan advantageous laser wavelength which may reduce or eliminate arequirement for special laser equipment operating at “exotic”wavelengths where the lasers are difficult to manufacture with highyield. The reflection may be estimated using a model wherein thethickness is a variable, and an estimate made to maximize thereflection, subject to other device constraints.

Thickness of the layers can be tuned to a wavelength in as much as thewavelength (or angle) can be tuned to the layers. Index of refractioncould be used to fine-tune over a limited range, but the range may notbe significant for small changes in index. Even with all thicknessesfixed by the process, the addition of a variable thickness tuning layeror layers with predetermined thickness could be used to significantlyaffect reflectivity of the whole stack. For example, a layer notconstrained by metallization requirements could be used as a precisionspacer between an upper and a lower stack portion. This could be a verypowerful tool for tuning the process with adjustment of perhaps only onelayer.

Physical Optics and Laser Processing of Multi-Level Devices

Other controllable laser characteristics may be exploited, alternativelyor in conjunction with wavelength selection, to provide furtherimprovements in the processing energy window. Reference 3 describes anadvantageous method and system for polarization control, includingdynamic polarization selection and computer control so as to align thepolarization with a link orientation (e.g., details shown in FIG. 4 andthe associated description in the reference). The polarization can beselected on the basis of the target coupling characteristics, the filmreflectance, or a combination thereof.

With a link dimension below the spot size, effects like diffraction,scattering, and edge reflection should be considered as physicalphenomena which can have either advantageous or detrimental resultsdepending upon the device geometry and beam characteristics. Likewise,at high energy density, non-linear absorption may affect results, withparticular concern of semiconductor material damage.

An additional important consideration with fine pitch (spacing) ofadjacent links and circuitry is collateral damage. Furthermore,functional circuitry in a plane of the layers must not be damaged. Withan increasing trend toward fine pitch and high density memory, thethree-dimensional structure of the device should be considered and mayaffect a choice of beam spatial and temporal characteristics. By way ofexample, FIGS. 4 a-4 c illustrate effects of reflection and diffractionassociated with sub-micron width link 10 resulting in truncated 43,44Gaussian beams 11, where the spot size (as measured at the 13.5% point)is wider than the link by varying degrees. The sketches arerepresentative of a diffraction limited beam waist at a near IRwavelengths. The central lobe is clipped by the link, which appears as anear field obscuration, resulting in transmitted beam portions which aretruncated 43,44. Energy which is not incident on the link may propagateat wide angles into the layers 49 which may be advantageous from thestandpoint of avoiding damage to the substrate 17 as shown in FIG. 1. Inany case, there will be some correlation of neighbor irradiance withspot size. Large spots with a relatively large depth of focus havereduced divergence and neighbor irradiance can be small, provided thatthe link spacing is large enough that the non-absorbed energy 43 of theincident beam impinging on an adjacent structure is weak, for instancecorresponding to level 44. With a higher N.A. and smaller spot size, thereflected beam diameter at the link location 46 is increased. There willbe a maximum value for some spot size 41,42. Then irradiance at aneighboring link 48 decreases as the reflected energy grows larger inarea.

Simultaneously there is an angular variation in internal reflection.Hence, the stack layer thickness can also effect the irradiance ofadjacent structures, including the internal structures 16 of FIG. 1.Furthermore, polarization variations with angle are expected to producevariations. FIGS. 6 a and 6 b illustrate by way of example geometric raytracing effects of internal reflections propagating over an extendedarea.

Similarly, as shown in FIG. 4 c, if a portion 45 of the laser beamincident on the edge of the link 46 is considered, the energy which isnot coupled into the link structure may also be scattered and/orspecularly reflected to the adjacent links 48. The inter-reflection 47occurs as a consequence of at least the link 46 physical edge profileswhich may be slightly curved or sloped.

An additional consideration is the three-dimensional spacing between aninner conductor layer 16 of FIG. 1, the beam waist 11, and the adjacentlinks 48 of FIG. 4 c. A large numerical aperture beam waist 11,producing the smallest spot size at the link, while diverging andreflecting in a manner so as to avoid significant interaction with theinner layer 16 is preferred. Examination of FIGS. 4 a-4 c suggest areduced spot size with controlled precision 3D waist positioning isexpected to reduce collateral damage by maximizing energy coupled intothe link. With high enclosed energy within the link and a low intensitytransmitted profile 44, edge reflection is minimized. The spatialprofile should also be selected subject to the constraint of only lowlevel, negligible interaction between the beam angular distribution at16.

It is preferred that the interaction mechanisms associated with aportion of the three-dimensional device structure be modeled forselection of at least a spatial pulse characteristic, such acharacteristic may be the N.A. and position of the beam waist.Preferably, the model will include an estimate of the irradiance seen byeach adjacent link structures 48, internal layer 16, and substrate 17.Whereas damage to adjacent link structures may be relatively apparentwith conventional microscopy, assessment of inner layer 16 and substrate17 damage may be considerably more difficult with the 3-D devicestructure.

With link widths below 1 μm, and pitch of a few microns, precise,sub-micron alignment is required to compensate for variations betweenwafers, and local variations within a wafer, and system tolerances(e.g., 300 mm wafer with 25 μm of topographical variation, and 5 μm ofmanufacturing tolerances, for instance). In accordance with the presentinvention, a precision positioning method and system is used torelatively position the beam waist so as to provide high laser energyconcentration at the link. Also, one important consideration forprecision positioning is predicting accurate (Xlink,Ylink) locationinformation. The prediction is subsequently used by a motion control andpositioning system to generate a laser output via trigger 2 at thetarget coordinates, during relative motion of the target 10 and laserbeam. A preferred embodiment includes a polarization insensitivescanning and detection system as described hereinbelow, wherein a regioncontaining an alignment target location is imaged to obtain referencedata. The target location is often covered by a dielectric layer ofSilicon Dioxide, Silicon Nitride, or other insulating material.Experiments have indicated that polarization insensitive detection isadvantageous to avoid spurious measurements. The results led to ahypothesis that that birefringence is introduced in the insulatinglayers by polishing or other process operations, which is manifested bypolarization variations in the reflected beam. These variations reducethe signal-to-noise ratio and appear to induce position distortion. Thedigital output data from each target location is used by an 8-parameterleast squares alignment algorithm to estimate and correct positioninformation affected by offset, angle, scale, orthogonality, andtrapezoidal variations over the wafer containing the links to beprocessed.

Given the variations in the received beam at the target location,concerns arise that process variations may affect layer opticalproperties near the target structure. Furthermore, in practice,variations occur in the thickness and reflectivity of the target andlayers, either over a wafer to be processed or from batch-to-batch.Measurement of the thickness and reflectivity is useful for processmonitoring, and can also be used to determine adjustments for the laserpower and wavelength to increase the energy window. For instance, anyvariation in the reflectivity of the link can affect the energy requiredfor processing. A preferred method and system for adaptive energycontrol is also described hereinbelow.

As dimensions of links and other microscopic structures continue torapidly shrink, those skilled in the art will appreciate the benefits ofmulti-parameter modeling. A model-based approach leads to selection andprecision control of the spatial and temporal characteristics of thelaser output, resulting in controlled three-dimensional interaction ofthe laser with complex multi-layer, multi-material structures.

Polarization Insensitive Detection and X, Y Reference Measurements

Commercial laser systems of the assignee of the present invention use abeamsplitter to pick off a portion of the reflected light from the worksurface (e.g., a multi-layer memory device) as the laser is relativelypositioned 152 over the alignment targets (e.g., fiducials). A blockdiagram of the subsystem is shown in FIG. 10. Thereflectance/transmission (R/T) split of the beamsplitter 150 depends onthe laser that is being used. In cases where the laser has low totalenergy and as much transmission as needed is necessary, the split of 90%transmission and 10% reflectance is made. This gives 90% going to thework surface on the way in and the 10% reflected is dumped. But thisonly picks off 10% of the reflected light, 90% of the reflected light istransmitted back down the laser path. When possible, the split 70/30 ismade. This gives less total energy to the work surface but gives higherreflected signal.

Regardless of the R/T split, the specification is the R/T for Spolarization=R/T for P polarization (within 5%). This is accomplishedwith a special dichroic coating, which produced good results. Becauseany polarization state can be thought of as a vector sum of S and P, thebeamsplitter works at the correct R/T ratio for any polarization.

This is important because switching polarization to any desired state isdone in the preferred link processing system to improve link cuttingefficiency. For example, co-pending U.S. application Ser. No.01/013,956, filed 13 Dec. 2001, a continuing application of U.S. Pat.No. 6,181,728 (Reference 3) and assigned to the assignee of the presentinvention reports results wherein a process window improvement occurswith polarization perpendicular to the link, particularly as the spotsize is reduced. The preferred polarization controller disclosed in the'728 patent is used to switch states.

The method and system of the present invention are advantageous whenthere are oxide layers over the targets to be scanned and measured. Theoxide layer may affect the polarization of the beam. This may happenbecause the oxide layer is stressed and creates birefringence. With thepolarization insensitive arrangement this is not a problem, no matterhow the polarization is changed one gets the same reflectance from thebeamsplitter and the same signal level. If a more typical polarizingbeamsplitter or simpler coating is used for the beamsplitter, thechanged polarization will result in a change in the reflected signal. Ifthe stress in the oxide layer varies, especially where it is over thetarget microstructure (it may be stressed because it is going over atarget edge) then the polarization may vary as the beam scans thetarget. Again, this is not a problem because of the coating. In thepolarizing beamsplitter case the reflected signal 151 measured at thedetector would vary because the polarization is varying at the same timethat one is trying to gather edge data, skewing the resultsuncontrollably and unpredictably.

This polarization insensitive technique is regarded as the most robustmethod and is preferred for measuring targets covered by at least oneoxide layer. However, other imaging and edge location methods may beused, but may require more complex measurement algorithms to accuratelymeasure the targets in the presence of multiplicative image noise.

Measurement with Anomalous Reflectivity Variations Cleaning with aPulsed Laser Beam

A typical alignment target 100 is depicted in the schematic drawings ofFIGS. 14 a and 14 b. The target 100 is typically covered with one ormore passivation layers, these may correspond to the layer 13 in FIGS. 1b and 1 c, but are not so restricted. During experiments with linkremoval on a multi-level, the preferred polarization insensitivemeasurement method was used to obtain X,Y target locations. However, itwas discovered that debris 1001 within the target area 100, possiblyfrom residual solder flux from nearby solder deposits (solder balls),significantly affected the reflected signals obtained with a detectorresulting in noisy profiles 101. The impact on the measurement wasmanifested as a large residual in the least squares fit algorithm usedto estimate location. In this illustration, the target area is shown asa positive contrast (e.g., higher measured intensity) region, but thoseskilled in the art will recognize that contrast reversal is acceptableprovided that the contrast between the target 100 and the background isadequate for measurement.

A pulsed beam with lower peak power was used to remove the debris. Anenhanced exemplary signal profile 102 (e.g., associated with relativelyuniform intensity and a mostly debris-fee region) was obtained as aresult of the cleaning operation, as shown in FIG. 14 b. Representativeenergies for cleaning were on the order of 0.01 μj, for instance, 0.005μj. This is well below the damage threshold of the materials, and wellbelow the typical energies used for removal of links 12.

In one embodiment, a single linear scan or a plurality of linear scans104 across the target 100 are used to obtain reflected intensity datawhich is analyzed statistically to measure fidelity, for instance bydetermining the % intensity variation or standard deviation. In anexemplary embodiment, data is taken along the line(s) 104 at about every0.001″. However, the sample space may be finer or coarser depending uponthe signal fidelity obtained. If the spacing is too fine, additional“texture noise” may be introduced. If too coarse, an edge contrast 107will be reduced or errors introduced by undersampling. If the variationis excessive, a cleaning operation is initiated with a pulsed beam.Preferably, the laser power is controlled with an acousto-opticmodulator (i.e., “energy control” of FIG. 13) which is a standard partof the laser processing equipment. The operation of the modulator forintensity control and pulse selection within a link blowing system isdescribed in more detail in U.S. Pat. No. 5,998,759 (e.g., Reference 4,col. 7 and the associated drawings). Those skilled in the art willrecognize that such modulators provide for intensity control over a widedynamic range, e.g., 100:1. A relatively simple user interface canprovide for operator interaction to initiate operation, based on“pass/fail” or other criteria.

In another embodiment, the linear scan(s) may be done automatically andthe cleaning operation performed at each measurement location.

In a preferred arrangement, only an adjustment of the energy will beneeded, and other system parameters unaltered during the cleaningoperation or as a result of cleaning. Those skilled in the art ofmeasurement will be able to make various adjustments in the systemparameters based on correlation of the results with other processparameters.

In a preferred arrangement, the cleaning operation will be applied onlyto scanned regions as needed. In one arrangement, the process isiterative with a measurement goal of obtaining suitable residuals in theleast squares fit algorithm. If the residuals are above a designatedvalue, scans of at least one region are obtained and cleaning occurs. Insome cases, it may be desirable to adjust the positions of the scanlines (e.g., if cleaning is difficult). A fidelity measurement (e.g.,contrast, standard deviation) may be used to guide the cleaningoperation. Preferably, no more than one pass will be required.

It is to be understood that numerous arrangements could be used topractice the cleaning invention. For instance, an array camera could beused with different wavelength illumination to identify regions ofnon-uniform intensity. These regions could then be designated forcleaning. Those skilled in the art of optical measurement will be ableto implement such arrangements, and such arrangements are within thescope of the present invention.

Reflectivity Measurement and Power Adjustment—Case 1: Single Wavelength

The above discussion related to a preferred measurement method andsystem for locating and measuring X,Y reference locations. An additionaloption to further improve the process energy window is measurement andcontrol concept to adjust the laser energy and power as required by thematerial to be processed. If the reflectivity is high, then the energyis to be increased to compensate for these reflection losses. If thereflection is low, then the energy and power is to be decreased sincemore energy is being coupled into the workpiece or targetmicrostructure. There are a number of ways that one can adjust thispower and energy. The simplest is to measure the reflectance from thesurface and adjust the energy and power control for optimum energycoupling.

Light interference between metal and oxide layers can greatly affect thereflection and hence the absorption in the metal links (see FIGS. 11 and12). Even though the process engineer tries to optimize the absorptionin the link by designing the best oxide thickness, the necessarythickness tolerance is difficult to control. Typically, the thickness ofa layer may vary by 10% and there may be several layers of oxide betweenthe top layer and the metal layer to be processed.

If the thickness and index refraction over the link could be determined,then the energy required to process the link could be calculated andadjusted accordingly. There are two methods of determining the opticalconstants of a film. These are ellipsometry and spectral analysis.Ellipsometry uses the change in polarization as a light beam eithertransmits or reflects from a surface. The amount of change inpolarization determines the index of refraction of the material andthickness of the material that the light beam traverses. Thespectrometric method measures the reflection from a surface at differentwavelengths to determine the same optical constants. In commercialversions of the spectrometer, the reflected light is sensed at 256different wavelengths and calculations made on thickness, index ofrefraction and extinction coefficient (absorptivity) of the layers tovery high accuracy.

Another method is to measure the reflectance at two differentwavelengths and calculate the thickness of the oxide. If the index ofrefraction of the oxide used for the device could be measured, then thereflectance and hence the fraction of the laser radiation absorbed overthe link can be calculated. Knowing this absorption, the optimum laserenergy to remove the link can be programmed into the laser system. Thissecond method is more accurate for thin film trimming where the materialto be trimmed is thin and some of the energy is transmitted through thefilm.

The implementation of the thickness measurement and energy control is asshown in FIG. 13. The laser 160 used to remove the link provides one ofthe laser wavelengths for the thickness measurement. The energydelivered to the part is controlled by an acousto-optical modulator(i.e., “energy control”) 161 as shown in FIG. 13 and is reduced to alevel to measure the reflectance without damaging the part. The otherwavelength to measure the reflectivity can be provided by a red laserdiode (i.e., 670 nm diode) 162 added into the optical path as shown.Beamsplitters 166,167 (e.g., dichroic mirrors) are generally used totransmit the two wavelengths to the device surface and to direct thereflected beams to the photodiode detectors 164,165. The reflectance canbe monitored by the two photodiodes 164,165 as shown in FIG. 13. Fromthe reflectance intensity of the two photodiodes (i.e., the 670 nm diodeand 1047 nm detectors) and knowing the index of refraction of the oxidelayers, the thickness of the oxide is uniquely determined. Once thethickness and the index of refraction is known, then the absorption inthe link material can be calculated and the optimum energy programmedinto the acousto-optic energy control device by the computer.

For the highest accuracy, the size of the spot and the link dimensionscan be used in the calculation. Referring to FIGS. 4 a and 4 b, one seesthat there is some energy that will fall off the link and therefore thedifference in the reflected light that does not fall on the link has tobe calculated. Hence, two measurements have to be made to accommodatefor the reflected energy that is not covered by the link. Thesemeasurements can be made on each die if required and the energy perpulse can be varied as the thickness of the oxide varies across thewafer. Alternatively, the method could be selectively applied on awafer-by-wafer basis for process monitoring, for instance. Thistechnique will reduce the requirement to use a laser processing energythat is on the high side to account for the variations in absorption inthe link due to interference effects.

Reflectivity Measurement and Power Adjustment—Case 2: Tunable orAdjustable Wavelength

The process energy window may be improved in certain cases by adjustingthe wavelength over a range wherein the coupling of energy to the targetis improved, the stack reflectance is increased by way of theinterference effect, or where the substrate reflectivity increases.Special solid state tunable lasers—Optical Parametric Oscillators (OPO),Raman, or other tunable lasers may be used provided that power andrepetition rate requirements are met for a given application. Forexample, parametric oscillators may be used which are of fixedwavelengths, that use 2 or 3 crystals at the same time. Under certaincircumstances, tunable lasers are operable. Published U.S. patentapplication 2001-0036206 describes a tunable laser diode having a 40 nmrange developed for the telecommunications industry (i.e. 1.55 μmwavelength). Standard OPO lasers provide for high power and narrowpulses but generally a very slow rep rate, but may be suitable forcertain applications. However, 10 KHz versions have been demonstratedand proposed for 20 KHz repetition rates. U.S. Pat. Nos. 6,334,011 and5,998,759 (Reference 4), and U.S. Pat. No. 6,340,806 (Reference 6)disclose various combinations of shifters. As disclosed in the '759patent, Fosterite lasers have a tunable region that essentiallystraddles the absorption edge region of silicon, and can permitoperation both beyond and below the absorption edge of silicon. At thepresent state of the art, they do not appear to be as efficient as theymay become in time. As materials and improvements are being continuallydeveloped in the laser field, it is within the invention to use suchdevices and obtain corresponding benefits for processing. For instance,the multilayer thickness and reflectivity measurements may be extendedto select a wavelength range which will provide for an improved energywindow.

Application to a Cu Link with a Single Layer Between the Substrate andthe Link

It should be noted that the above teachings can also be selectivelyapplied to conventional link structures (see FIG. 2-B), for instanceprocessing of high reflectivity copper links separated from thesubstrate by a single layer dielectric layer. Production trends arepushing away from polysilicon structures and toward metal structures ofAl and Cu, which poses on-going challenges for link processing systemsto avoid reliability problems and to increase yield. As discussed above,many Cu-based devices have a multi-layer stack wherein substrate andstack damage can be avoided with wavelength selection, spatial beamshaping, or temporal shaping in accordance with the above teachings.However, some manufacturers etch all the dielectrics under the copperlink and build the fuse on a single layer dielectric, with no SiN layersbetween the link and substrate. With conventional laser processing, thelikelihood of substrate damages increases due to the high power requiredfor Cu processing.

In certain cases processing with multiple pulses (“double blast”) hasbeen used to process metal fuses. However, there is generally athroughput problem for the double blast approach because two passes arerequired in present on-line memory processing systems. Simulated resultsand experiments indicate a second blast may open the link completelyeven if 1st blast failed, despite extended time between first and secondblasts. In specific cases improved yield was reported. According to thesimulation results, double blast with 50% energy of a single blastenergy was very interesting; it was observed that the Si substrate actsas a heat sink and cools down very fast. As shown in FIG. 16, theresults indicated only 10 to 20 ns are needed for the Si substrate 201to stabilize to room temperature. The copper target 202 recovery wasmuch slower indicating a significant differential thermal property. Thesecond pulse will also clear debris at cut site resulting in an “opencircuit”. It is estimated that about 60-70% of the energy used in a“single blast” is needed for each pulse of “double blast.” The pulseenergy may be varied with each pulse. In this example, the pulse delaywas 50 ns, but it is clear that a much shorter delay may be possible.

In one embodiment, a delay line arrangement of FIG. 15 a may be used toavoid any delay in throughput. For example, with a preferred positioningsystem of the '118 patent (i.e., Reference 2) assume about 150 mm/secfor fine stage speed movement. With 30 ns between two pulses, the changein beam position at the link location would be only 0.0045 um which isnegligible. In an optical delay line (FIGS. 15 b and 15 c, forinstance), 9 meters of extended path in air for the beam will delay 30ns for the second pulse. Alternatively, as shown in FIG. 15 a, a secondlaser could be used with a 30 ns or other controllable delay between thetrigger pulses, and the trigger delay may be generated with aprogrammable digital delay line. The temporal pulse shape may be a fastrising, square pulse (as was used in the simulation) generated with aseed laser diode, for instance.

Numerous options for generating the pulse combinations may beimplemented based on the teachings herein. For example, at least onepulse may have a duration of greater than a few picoseconds to severalnanoseconds. The pulses may be amplified mode locked pulses. At leastone pulse may generated with a q-switched microlaser having a pulsewidthless than 5 nanoseconds. At least one pulse may propagate along a secondoptical path whereby the pulse delay is determined by a difference inoptical path length as shown in FIGS. 15 b,c. Multiple laser and/oramplifiers may be used as shown in FIG. 15 a.

As shown in FIG. 18, the generated pulses 275 may have a repetition rateand a corresponding temporal spacing approximately equal to or shorterthan a pre-determined delay (e.g., 60 MHz mode locked system) and amodulator is used to select the at least second pulse irradiating themicrostructure or groups of pulses 276. U.S. Pat. No. 5,998,759 (e.g.,Reference 4, col. 7, and the associated drawings) teaches the use of amodulator to allow pulses to irradiate a link on demand. At very highspeed repetition rates an electro-optic modulator is preferred.

Additional optics may be used to spatially shape at least one of thedelayed pulses, prior to combining for instance. For instance, as shownin FIG. 17, a first pulse 210 may be an elliptical or circular Gaussianspatial shape, or a top hat along the length of the link. The secondpulse 212 may have a different aspect ratio, or may be a special form ofa “cleaning pulse” wherein the central zone of the spot is attenuatedwith an apodizing filter or effectively removed with a centralobscuration. In such a case, the energy will be concentrated at the linkperiphery to remove debris 211 around the link location resulting fromprocessing with the first pulse, thereby completing the processing 213.(For clarity, this “on-the-fly” link site cleaning step is to bedistinguished from the “cleaning for measurement” method describedabove). Reference 1 provides at least one example of beam shaping forlink blowing applications, wherein a uniform distribution rather thanGaussian spot profile is disclosed.

In certain cases, the relative motion between the microstructure and thelaser beam may be significant between the pulses, e.g., greater than 25%of the spot size. This may be the result of a slower repetition rate(with increased pulse energy), faster motion speed, a longerpre-determined delay, or decreased target area. For example, anultrashort or other short pulse laser system with amplified pulses withoutput energy in the range of several microjoules-millijoules may have a100 KHz-10 MHz repetition rate whereas a system with 10-40 nanojouleoutput may have 50 MHz repetition rate. In the former case, a highspeed, small angle beam deflector may be used to compensate for themotion and deflect a delayed pulse to substantially irradiate the firstmicrostructure at the slower repetition rate during relative motion 258.

In one embodiment generally illustrated in FIG. 19, the deflector wouldbe operatively coupled to the relative positioning system controller 251in a closed loop arrangement. The deflector is preferably solid stateand may be a single axis acousto-optic device which has a very fast“retrace”/access time. Alternatively, a higher speed electro-opticdeflector (e.g., a gradient index reflector or possibly a digital lightdeflector) may be used. The time-bandwidth product (number of spots) canbe traded for response time on an application basis. The deflector wouldpreferably be used for intensity control and pulse gating/selection, astaught in Reference 4 (col. 7, and associated drawings). Alternatively,an electro-optic modulator may be used with a separate acousto-opticdeflector operated in a “chirp mode” 252 (e.g., linear sweep as opposedto random access mode) and synchronized (triggered) 253 based on thepositioning system coordinates 254. The positioning system coordinatesare, in turn, related to the time at which the laser pulses are gated bythe modulator to irradiate the same single microstructure 256 at timest₁, t₂, t₃ corresponding to the selected pulses 259 during relativemotion 258.

In yet another embodiment, a single laser pulse is used to blast up totwo links at one time (e.g., no, one or two links). Referring to FIG.20, two focused spots 306,307 are formed on two links by spatiallysplitting the single collimated laser beam 310 into two divergingcollimated beams 309. The use of acousto-optic devices for spatiallysplitting beams in material processing applications is known in the art.For example, patent abstract JP 53152662 shows one arrangement fordrilling microscopic holes using a multi-frequency deflector havingselectable frequencies f₁ . . . f_(n).

A laser 300 is pulsed at a predetermined repetition rate. The laser beamgoes through relay optics 302 that forms an intermediate image of thelaser beam waist into the acoustic optic modulator (AOM) aperture. TheAOM 303, which operates in the Bragg regime, preferably is used tocontrollably generate the two slightly diverging collimated first orderdiffraction laser beams and control the energy in each beam. The AOM isdriven by two frequencies, f₁ and f₂ where f₁=f_(0+Δf) and f₂=f_(0−Δf)where Δf is a small percentage of the original RF signal frequency f₀.The angle between the two beams is approximately equal to the Braggangle for f₀ multiplied by 2(Δf/f₀). The AOM controls the energy in eachof the laser beams by modulating the signal amplitudes of two frequencycomponents, f₁ and f₂, in the RF signal and making adjustments for beamcross-coupling.

After exiting the AOM, the beams go through the beam rotation controlmodule 313 to rotate the beam 90 degrees on axis with links orientatedin either the X or Y. In one embodiment, a prism is used for thisrotation, though many rotation techniques are well known as described inthe regular U.S. application noted in the Cross-Reference to RelatedApplications section.

Next, the beam goes through a set of optics to position the beam waistand set the beam size to be appropriate for the zoom optics and theobjective lens 305. Note, the zoom optics also modify the angle betweenthe two beams, therefore the angle between the two beams exiting the AOMhas to be adjusted depending on the zoom setting to result in thedesired spot separation at the focal plane. Next, the laser beams enterthe objective lens 305 which provides a pair of focused spots 306, 307on two links. The two spots are separated by a distance that isapproximately equal to the focal length of the lens times the anglebetween the two beams. In one exemplary embodiment, a 80 MHz AOM centerfrequency with a sweep range of about 2.3 MHz (77.7-82.3 MHz) may beused to produce a spot size of about 1.8 μm on a pair of adjacent linksspaced apart by about 3 μm. As mentioned earlier, these links may have adimension on the order of a laser wavelength (e.g., 1 micron) which, atvery high speed operation, require the precision positions of the laserbeam and microstructure.

Summary of Some General Aspects of Invention

In summary, one aspect of the invention is a method of selectivematerial processing of a microscopic target structure with a pulsedlaser beam. The target structure is separated from a substrate by aplurality of layers which form a multi-layer stack. The targetstructure, layers, and substrate have dissimilar thermal and opticalproperties. The method includes generating a pulsed laser beam with anenergy density; irradiating the target structure with at least onepulse. Undesirable changes to the stack structure and substrate areavoided by selection of at least one pulse characteristic.

A portion of the stack may be irradiated with the laser beam during theprocessing of the target structure, yet undesirable damage to thelayers, substrate, and functional circuitry in a plane of the innerlayers is avoided.

Undesirable damage of the stack structure includes cracking, induced bythermal stress, of inner dielectrics. Undesirable damage to inner layerconductors of the stack includes thermal damage caused by irradiation.Undesirable damage to the substrate may arise from laser irradiation andresulting thermal diffusion.

The dielectric layers may include Silicon Nitride or Silicon Dioxide.The substrate may be Silicon.

The target structure is preferably copper, and may have thickness orwidth below one micron, with a dimension at or below wavelengths ofvisible light. Alternatively, the target structure may be a metal link,for instance aluminum, titanium, platinum, or gold.

An aspect of the invention is selection or control of the spatial andtemporal beam characteristics of the pulse, which allows the targetstructure to be cleanly processed while avoiding undesirable damage tothe layers, substrate, and functional circuitry in a plane of the innerlayers.

A temporal characteristic of the pulse is the pulse shape. The pulseshape includes a rise time fast enough to efficiently couple laserenergy into the target, a duration sufficient to cleanly remove aportion of the target structure, and a fall time fast enough to avoidundesirable damage caused by subsequent optical transmission. Apreferred pulse rise time for link processing is less than 1 nanosecond(ns) to about 2 ns. A preferred duration is less than 10 ns. A fall timeof less than 3 ns is preferred. The pulse shape may be substantiallysquare, with ringing or variation between the rising and falling edgesof about +−10%. A single pulse or multiple pulses in the form of a rapidburst may be used. Alternatively, a series of q-switched pulses spacedapart in time, with varying output power if desired, may be combined toform a pulse shape having a fast leading edge with high peak power,followed by a second pulse with lower power. In yet another embodimentof the present invention the q-switched pulses may have approximatelythe same output power and combined to produce a substantially squarepulse shape.

Another temporal pulse characteristic is the pulse power at the leadingedge. If the irradiance on the target structure is greater than about10⁹ W/cm², the reflectivity of the target structure is reduced andcoupling of the laser energy is improved.

A fast rising pulse characteristic avoids undesirable damage of adielectric stack of a memory device having a metal target structure.Cracking of the upper corner occurs during the pulse duration whichlowers the stress on the lower corner adjacent to underlying layers ofthe stack.

A spatial characteristic of the beam is the irradiance profile at acontrolled beam waist position. The irradiance profile may approximate acircular Gaussian beam, an elliptical Gaussian beam, a rectangularprofile in one direction and Gaussian in the orthogonal direction. Thebeam may be nearly diffraction limited. A spatial shape and beamnumerical aperture may be selected to control the interaction of thepulsed laser beam with the target and underlying structures of the 3Ddevice structure to avoid undesirable damage. The material interactionmay further be controlled by precision positioning of the beam waist ofthe pulsed laser beam. The numerical aperture and beam shape may beselected so the spot size and link size are substantially matched in atleast one dimension.

One aspect of the invention is a method of selection of a pulsecharacteristic based on a model of pulse interaction within a portion ofthe three-dimensional device structure. The three-dimensional deviceincludes a target structure, stack, and substrate with a dissimilaroptical property. A series of structures are disposed at a predeterminedspacing to form an array, with at least one structure not designated asa target structure. A specification may further include informationregarding the material and spacing of functional circuit elements in aplane of the stack. The method includes determining the opticalpropagation characteristics of a portion of an incident pulsed laserbeam which is not absorbed by the target structure. The method furtherincludes specifying a laser pulse characteristic to avoid undesirabledamage to any non-target structures, stack, and substrate.

The interactions mechanisms which result in selection of a pulsecharacteristic include reflection from the target surface, layer surfaceand internal reflections, polarization, interference effects, near fielddiffraction, scattering and absorption or a combination thereof. Athermal model may be used in conjunction with an optical model.

The energy in a pulse used for processing a copper link target structureof a semiconductor memory device may be in the range of about 0.1-5microjoules. The energy density corresponds to an area of the irradianceprofile of the beam waist. The area may be in the range of less than 20square microns, and preferably less than 10 square microns.

Another controllable laser pulse characteristic is polarization. Thepolarization may be controlled or selected based on the relativereflectance of the layers and optical coupling of laser energy into thetarget structure at a wavelength.

A wavelength of the laser pulse may be selected based on the reflectanceof the multi-layer stack (interference effect). The preferred wavelengthcorresponds to a spectral region where the stack reflection issubstantial, for example 60%, and where the internal transmission ofwithin a layer of the stack is high, approaching a maximum. Shortwavelengths are preferred for maximum control of the spatialcharacteristics of the beam (for example, the smallest achievable beamwaist with an option for controllably selecting a larger beam waist anddepth of focus). The laser wavelength may be fixed, or may be variedwith wavelength shifting or harmonic generation. A measurement of thethickness or reflectance may be used to select or adjust the wavelength.

In at least one embodiment, the target structure may be substantiallyreflective at the laser wavelength. The laser wavelength may be belowthe absorption edge of the substrate and correspond to an absorbing orreflecting region. The laser wavelength is above the absorption edge ofthe dielectrics layers of the stack, and corresponds to a substantiallymaximum transmitting region.

A selected wavelength corresponds to the near UV, visible and near IRspectrum, from below 0.4 μm to about 1.55 μm. The lower limit may bedetermined by the absorption of a layer. With silicon substrates, bothabsorption and reflection increase at shorter wavelengths. For SiliconDioxide and Silicon Nitride, the internal transmission and singlesurface reflectance are substantially constant throughout the visibleand near IR ranges. The upper limit corresponds to a range of preferredlaser wavelengths of laser diodes, optical amplifiers. An amplifieroutput may be either wavelength preserved or Raman shifted.

Another aspect of the invention is a method of selective materialprocessing of a microscopic target structure of a multi-material,multi-layer device with a pulsed laser beam. The target structure,layers, and substrate have a dissimilar thermal and optical property.The beam has a focused beam waist with a centerline. An alignmentpattern is included at one of a plurality of predetermined measurementlocations associated with the device. The alignment pattern is coveredby at least one layer. The target structure is separated from asubstrate by a plurality of layers which form a multi-layer stack. Themethod includes measuring the position of the alignment target in atleast one dimension; predicting the relative location of the targetstructure and centerline based on the measurement; inducing relativemotion between the target structure and the centerline based on themeasurement; generating a pulsed laser beam with an energy density;irradiating the target structure with at least one pulse. Undesirablechanges to the stack structure and substrate are avoided with byselection of a pulse characteristic.

The measurement of a position may include a method and system forpolarization insensitive detection to avoid spurious measurementsresulting from reflected signal variations. The signal variations mayresult from process induced optical characteristics, includingbirefringence.

The relative location of the target structure, beam waist, andcenterline may be predicted based on multi-parameter least squares fit.

A cleaning process may be used to enhance data used for measurement byremoving contaminants which produce multiplicative variations(reflection noise).

Three-dimensional (depth) measurements may be done using the alignmenttarget, wafer, or other suitable material. The measurement may be usedto predict the relative location of the target structure relative to thebeam waist, the beam waist being located along the centerline of thepulsed laser beam. A surface may be estimated from the three-dimensionalmeasurements. A numerical offset may be introduced to compensate for adepth difference between a measurement location and the targetstructure, based on the thickness of the stack.

An aspect of the invention includes measurement of the layer thicknessor reflectivity at a location, and use of the measurement to control apulse characteristic. The pulse characteristic may be the pulse energy,pulse width, or wavelength. The location may be a single location on thedevice or a plurality of locations.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims.

1. A method of laser processing a multi-level, multi-material deviceincluding a substrate, a microstructure and a multi-layer stack, thestack having at least two inner dielectric layers which separate themicrostructure from the substrate, the method comprising: generating apulsed laser beam having a predetermined wavelength less than anabsorption edge of the substrate, the substrate having a relatively highabsorption coefficient at the predetermined wavelength and the stackhaving a low absorption coefficient at the predetermined wavelength andincluding at least one laser pulse wherein at least reflections of thelaser beam by the layers of the stack substantially reduce pulse energydensity at the substrate relative to at least one other wavelength andwherein the at least one laser pulse has a duration of greater than afew picoseconds to several nanoseconds; and processing themicrostructure with the at least one laser pulse wherein pulse energydensity at the microstructure is sufficient to remove the microstructurewhile avoiding damage to the substrate and the inner layers of thestack.
 2. The method of claim 1 wherein the predetermined wavelength isa near UV wavelength above the absorption edge of the stack.
 3. Themethod of claim 1 wherein the substrate is a silicon substrate and themicrostructure is a metal link.
 4. The method of claim 3 wherein themetal of the link comprises copper.
 5. The method of claim 1 whereinpulse energy density at the substrate is further reduced with apredetermined characteristic of the at least one pulse.
 6. The method ofclaim 5 wherein the predetermined characteristic is based on a model oflaser pulse material interaction.
 7. The method of claim 6 wherein themodel is a thermal model.
 8. The method of claim 6 wherein the model isa multi-parameter model.
 9. The method of claim 1 wherein the step ofgenerating includes the step of shifting the wavelength of the laserbeam from a first wavelength to the predetermined wavelength wherein thepredetermined wavelength is based on material characteristics comprisingat least one of: (1) coupling characteristics of the microstructure, (2)multi-layer interference, and (3) substrate reflectivity.
 10. The methodof claim 1 wherein the materials of the at least two inner dielectriclayers are different.
 11. The method of claim 2 wherein thepredetermined wavelength is about 0.35 um.
 12. The method of claim 5wherein the predetermined characteristic is a spatial characteristic.